The present invention relates to fracturing operations and, more particularly to, compositions and methods related to proppant placement using an expandable material.
Fracturing (e.g., hydraulic fracturing) is a technique in which fractures are propagated in a subterranean formation by the introduction of a pressurized fluid. Often, fracturing operations are performed for the purpose of extracting certain materials such as petroleum, natural gas, coal seam gas, and the like. Hydraulic fracturing can be used to increase or restore the rate at which such materials can be produced from a subterranean formation.
In a typical hydraulic fracturing operation, a proppant (also known in the art as a “propping agent”) is suspended in a portion of a treatment fluid, which may then be transported and deposited in fractures within the subterranean formation. The proppant serves to prevent the fractures from fully closing so that conductive channels are formed through which produced hydrocarbons can flow. The degree of success of a fracturing operation depends, at least in part, upon the resultant fracture porosity and conductivity once the fracturing operation is stopped and production is begun. Typical hydraulic fracturing operations place a large volume of suspended proppants into a fracture to form a relatively homogeneous proppant pack within the fracture. The porosity of the resultant packed, propped fracture is related, at least in part, to the interconnected interstitial spaces between the abutting proppant particulates.
An alternative fracturing approach involves placing a much reduced volume of suspended proppants in a fracture to create a fracture having high porosity, permeability, and/or conductivity. The reduced volume of proppants may be consolidated to form individual aggregate structures that are often referred to as “proppant aggregates” or “proppant pillars.” As used herein, the term “proppant aggregates” and related terms, such as “proppant pillars,” refers to a group of proppants that remains a coherent body when placed into a fracture. The term “proppant pillar” refers to proppant aggregates that have substantially pillar-like structure or placement within the fracture. The proppant aggregates preferably remain coherent bodies that do not become dispersed into separate proppant particles without application of significant shear.
Achieving a heterogeneous proppant placement, such as a pillar placement, typically involves pumping different types of slurries or fluids in discrete intervals. This can provide higher conductivity fractures than those obtained from conventional treatments, and may increase fracture conductivity and fracture porosity by replacing the standard, homogeneous proppant pack with a heterogeneous proppant pack. In some cases, the slurries contain proppants coated with adhesives, such that the proppants exhibit a sticky or tacky character and have a tendency to cluster or aggregate (i.e., form proppant pillars). Proppant pillars (sometimes referred to as “aggregates,” or “posts”) are placed at intervals throughout the fracture. These pillars have sufficient strength to hold the fracture partially open under closure stress. The open space between pillars forms a network of interconnected, open channels available for flow of fluids into the wellbore. This results in a significant increase of the effective hydraulic conductivity of the overall fracture.
However, there are issues that limit the usefulness of heterogeneous proppant placement. For example, proppant settling is especially problematic in heterogeneous proppant placement because reduced volumes of proppants are typically used. Proppant settling may lead to closure of a portion of the fracture (typically the top portion), which can lower the conductivity of the propped fracture and result in proppant aggregation rather than the desired discrete pillars. Moreover, one of the main technical challenges of heterogeneous proppant placement is the aggregation or formation of proppant pillars from individual proppants and/or proppant clusters.